1. Field of the Invention
The present invention relates to an ultrasonic endoscope, both an objective optical system for optical observation and an ultrasonic transducer being incorporated into the tip of the endoscope.
2. Description of the Related Art
Endoscopic ultrasonography (EUS) is a combination of endoscopy and ultrasonography, a small ultrasonic transducer (ultrasonic probe) being incorporated into the tip of an endoscope. Two different instrument types of EUS are currently available: a linear-type echo endoscope (providing an ultrasonic view which extends parallel to the shaft axis of the instrument) and a radial-type echo endoscope (providing a 360.degree. ultrasonic panoramic view which extends perpendicular to the shaft axis of the instrument). These types each have merits and demerits and are thus selectively used depending on the circumstances.
EUS is used for taking ultrasonic tomograms or plane sectional images of, e.g., a viscera or part of a viscera which is located at the back or behind a mucous membrane. When the EUS is used in an endoscope, a needle (an aspiration needle or an injection needle) which is introduced into the body through a treatment tool insertion channel is penetrated into a target part through a mucous membrane. In this case, when the radial-type echo endoscope is used, the course of the needle cannot be monitored since only a small part of the whole image of the needle appears in any plane sectional images, so that the penetration depth cannot be monitored. Therefore, an operation using the radial-type echo endoscope cannot be said to be performed with safety. On the other hand, when the linear-type echo endoscope is used, the course of the needle can be monitored since the needle is guided in an ultrasonic scanning surface therealong, so that the penetration depth can be clearly monitored. However, since any other plane sectional images of the surroundings of the penetrated portion cannot be monitored at all, while the plane sectional image of the penetrated portion is monitored, it is difficult to make sure if the penetrated portion is actually an appropriate portion to be penetrated by the needle.